1. Field of the Invention
The present invention relates to processes and compositions for electroless metallization and related articles of manufacture and, more particularly, the invention relates to the use of substrate chemical groups capable of ligating with a variety of electroless metallization catalysts, including tin-free catalysts, and selective electroless plating through the use of such ligating groups.
2. Background Art
Electroless metallization procedures typically require multiple and complex processing steps. See, for example, reviews of electroless plating in C. R. Shipley, Jr., Plating and Surface Finishing, vol. 71, pp. 92-99 (1984); and Metal Finishing Guidebook and Directory, vol. 86, published by Metals and Plastics Publications, Inc. (1988), both incorporated herein by reference. One typical procedure for metallization of polymeric substrates employs a colloidal palladium-tin catalyst in the following sequence: (1) pre-cleaning the substrate surface; (2) microetching, for example with a chromic-based solution; (3) conditioning the etched substrate surface; (4) adsorption of the palladium-tin catalyst onto the conditioned surface; (5) treatment with an accelerator to modify and activate the absorbed catalyst; and (6) treatment with an electroless plating solution. See, for example, U.S. Pat. Nos. 4,061,588 and 3,011,920. A number of fundamental studies have been performed on this and related electroless procedures. See, for example, J. Horkans, J. Electrochem. Soc., 130, 311 (1983); T. Osaka, et al., J. Electrochem. Soc., 127, 1021 (1980); R. Cohen, et al., J. Electrochem. Soc., 120, 502 (1973); and N. Feldstein, et al., J. Electrochem. Soc., 119, 668 and 1486 (1972).
While the exact composition and structure of such a Pd/Sn catalyst have not been confirmed, and the detailed mechanism by which a Pd/Sn colloid adheres to a substrate is not fully understood, the following is known and/or currently postulated. A palladium-tin electroless catalyst typically is generated by mixing multi-molar stannous chloride and a palladium chloride in an acidic aqueous solution containing excess chloride ion. Sn(II) reduces the Pd(II) species, likely via an inner-sphere redox reaction in a Pd/Sn complex, resulting in a colloidal suspension with a dense metallic core within a less dense tin polymer layer. The central portion of the colloid is composed of an intermetallic compound of stoichiometry reported to be Pd.sub.3 Sn. This inner core is believed to be a cluster containing up to 20 atoms with palladium principally in the zero and +1 oxidation states. This inner core is the actual catalyst in the initial metal reduction that leads to electroless metal deposition.
Surrounding this core is a layer of hydrolyzed stannous and stannic species that forms an outer shell of oxy- and/or hydroxy-bridged oligomers and polymers together with associated chloride ions. This layer is known as beta-stannic acid. The composition of the colloidal suspension contains a high concentration (multi-molar excess) of stannous ions relative to Pd which continue to hydrolyze and form higher oligomers on the outer surface of the initially formed colloidal particles. Consequently the thickness and degree of polymerization of the outer tin shell changes over time. The resultant colloidal particle has a net negative charge.
Adhesive properties of the outer polymeric outer shell attach the catalyst to the substrate to be plated, known in the art as the activation process. The negative charge of the outer tin shell prevents aggregation of the colloids permitting individual attachment to the substrate. The reducing power of the Sn(II) acts as an anti-oxidant and protective layer that maintains the catalytic core in the low valent Pd state necessary to initiate plating. Activation is followed by an acceleration step whereby the catalyst core is exposed. Acceleration can be achieved by a variety of means, for instance by "subtractive" type means of dissolving the stannous protective layer at high chloride ion concentrations to form soluble SnCl.sub.4.sup.2-, or by oxidizing the shell to the more soluble Sn(IV) by exposure to oxygen from the ambient. "Additive" type acceleration sequences are also known. For example, European Patent Application 90105228.2 discloses the application of an acidic solution of PdCl.sub.2 to the intact adsorbed colloid. The stannous polymer layer of the particle reduces the palladium ion in situ to form a metallic Pd deposit on which plating can occur.
After activation, the substrate is immersed in an electroless plating solution. A typical electroless metal plating solution comprises a soluble ion of the metal to be deposited, a reducing agent and such other ligands, salts and additives that are required to obtain a stable bath having the desired plating rate, deposit morphology and other characteristics. Common reductants include hypophosphite ion (H.sub.2 PO.sub.2.sup.-), formaldehyde, hydrazine or dimethylamine-borane. The reductant reacts irreversibly at the catalyst core to produce an active hydrogen species, presumably a palladium hydride. The surface hydrogen is also a potent reductant which transfers electrons to the soluble metal complex in the bath and produces a metal deposit on top of the catalyst, which eventually covers the core sufficiently to block access to the external solution. For certain deposits, such as copper, nickel and cobalt, the nascent layer can itself become "charged" with hydrogen and continue to reduce metal ion to metal, leading to "autocatalytic" build-up of an electroless deposit onto the activated surface. In a competitive reaction, surface hydrogen atoms combine to evolve H.sub.2 gas. This latter reaction has never been completely suppressed. Therefore, not all available reducing equivalents in the electroless bath can be used for metal deposition. For a properly catalyzed surface, the choice of electroless metal plating solution is determined by the desired properties of the deposit, such as conductivity, magnetic properties, ductility, grain size and structure, and corrosion resistance.
Such a palladium-tin catalyst system presents a number of limitations. At a minimum three steps are required--activation, acceleration and plating. Often substrate pre-treatment and other additional steps are necessary to provide uniform plating. The colloidal catalyst also is readily oxidized and stannous ions must be replenished by regular addition of Sn(II) salts. Further, the colloid size may fix packing density thereby making difficult uniform plating of ultra-small objects, e.g. objects less than about 1,000 angstroms in size. Subtractive-type acceleration requires a precise and often difficult balance of exposing the palladium core without dissolving the portion of the stannous shell that provides adherence to the substrate surface. Further, substrate adhesion of a Pd/Sn catalyst has been found to be a relatively non-specific phenomenon. For example, the catalyst will only weakly adhere to a smooth photoresist coating, requiring a pre-etch step to provide a more textured surface and thereby increasing processing time and costs. For many situations, such as high resolution lithography, such pre-etching is not feasible. Further, a number of materials are "colloidophobic", i.e. materials to which a Pd/Sn catalyst does not adsorb. These materials include silica, certain metals and some plastics.
Recently, several electroless plating procedures have been reported, the procedures generally employing a palladium catalyst and a polyacrylic acid or polyacrylamide substrate coating. See, U.S. Pat. Nos. 4,981,715 and 4,701,351; and Jackson, J. Electrochem. Soc., 135, 3172-3173 (1988), all incorporated herein by reference.
A common method for producing a patterned metallized image includes use of a photoresist coating. In an additive metallization approach, photoresist is applied to a substrate surface; the resist is exposed to provide selectively soluble portions of the photoresist coating; a developer is applied to bare selected portions of the substrate surface; those selected portions are metallized; and the remaining resist stripped from the substrate surface. See, generally, Coombs, Printed Circuits Handbook, ch. 11 (McGraw Hill 1988), incorporated herein by reference. A print and etch procedure is a subtractive approach where in the case of circuit line fabrication, a copper layer is selectively chemically etched through use of a photoresist to define the circuit traces. For higher performance applications, it is crucial that circuit sidewalls be uniform and essentially vertical. Resolution limits exist with a print and etch sequence, however, which are inherent in the subtractive nature of this approach.